Isolated mammalian muscle spindles with the capsule removed in the sensory region are used to study the effects of ions and drugs on the receptor potential response to ramp-and-hold stretch. Microphotographic analysis of length changes is used, in addition to tension recording, to evaluate mechanical factors. Electron microscopy is employed to identify profiles seen under Nomarski optics. The aim of the research is an understanding of the ionic conductance changes produced by stretch deformation in this receptor and the mechanical transmission of force to the sensory terminals.